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Broadband suspended patch antennas with
ground taper transformer
N.U. Lau, C.M. Gregory and R. Sloan
A novel ground taper has been designed and then fabricated using a
laser cutter. The taper is used to match the transition between a
suspended microstrip line and a microstrip line. This transition is used
in the design of a 25 GHz suspended patch antenna deposited onto a
polyimide tape and yields an 18% hctional bandwidth.
Introduction: Conventional microstrip patch antennas in general have
a relatively narrow bandwidth of 1-5%. A thick substrate with a low
dielectric constant can normally be used to improve the bandwidth.
However, it can also generate significant surface wave modes and
deteriorate the performance of antenna radiation, especially in the
millimetre-wave range. Air substrates have been used in broadband
antenna designs [ 1 4 ] , and elimination of surface wave propagation at
millimetre-wave frequencies achieved [5, 61. In these designs, the
substrate underneath the patch was etched away, hence creating a
mixture of single- and double-layered layouts. Owing to the difference
in dielectric constant and substrate thickness, microstrip lines with the
same characteristic impedance will have different line widths over the
two different layers, hence it requires the use of a transition between
conventional microstrip and suspended substrate microstrip technologies. To avoid this, aperture-coupled feed mechanisms have been
implemented in designs [ 1,5,6] where the feed-line and antenna were
placed on two different layers. Nevertheless, there are antenna
requirements which are constrained by single planar microstrip circuit
layout, where it will not be possible to use the multilayer approach.
Ioannis et al. [4] designed a patch antenna with 5050% of air:silicon
substrates, and the feed-line was maintained at a constant width over
the composite substrate region, resulting in a mismatch on the
transition boundary. Despite the mismatch, an increase in bandwidth
by as much as 64% over conventional designs is still achievable.
Similar results are reported in [2] where a 12.5% of -1OdB
bandwidth was achieved using an air-filled cavity behind the microstrip line fed patch antenna.
500 Frn brass
150 flm
ground plane
polyimide tape
Design procedures: Air cavity: The distance between the edges of
the antenna and cavity was proposed to be at least twice the substrate
thickness (h) due to the presence of fringing fields [ 4 ] . There are
designs with the air cavity walls placed 6h away from the edge of
the patch [4, 61 where improvements in radiation efficiency are
reported. Here an intermediate value is taken and the air cavity was
created 4h away from the edge of antenna to avoid perturbation of the
EM-fields. Impedance matching: A novel ground-taper impedancetransformer (GT) is designed to match the antenna impedance to a
50 R feed-line. GT design utilises a smooth change of ground plane to
the width of the air cavity, while keeping the transmission line width
constant throughout the transition boundary. Changes in characteristic
impedance to the variation of the ground-plane spacing are first
studied; then an exponential curve taper is designed. The curve
taper is modified into a triangulated shape with double bend edges
to obtain the closest match to a curved taper, so that it is simpler to be
manufactured by laser cutting yet remains close to a curve taper. A
similar structure was first reported in [7] and has already been used for
the design of a microstrip/slot directional coupler [8], but is implemented here on a suspended patch, antenna design for the first time.
The analysis of the structure was performed by Agilent's momentum
and HFSS. Prototype: The suspended patch antenna structure is
difficult to fabricate monolithically, hence a simple and effective
structure is proposed to realise the design prototypes. A 150 pm
thick polyimide tape is used as substrate ( E , = 3) and the patch antenna
rests on a 500 pm-thick hollow brass ground plane (Fig. 1).
Or
I
-25 -
I
frequency, GHz
/
\
a
0m
XI
i
-
-10-
GT-Ant (E-plane)
B
2 -20
/f
GT-Ant (H-plane)
.-
- . - . -. . . . . . . . . .
.. .. . ..
, '.,
,.
I
;
4-
a
a
E -30
,.I,,.
.....
.%
3
E
E!
:_
._
",
-
.-.-.
..........
....
'\\
,,
-.
GT-Ant-Hplane-cross
-40
'
. . ,:
1
GT-Ant-Eplane-cross
-50
IO
-50
0
50
100
phase, deg
b
Fig. 2 Measurement for proposed antenna
a Measured input retum loss for GT-Ant and REF-Ant
b Radiation patterns for GT-Ant at 25 GHz
b
Fig. 1 Cavity backed patch antenna with ground taper transformer
transition
a Three-dimensionalview of patch antenna
b Top view of patch antenna
a=4.4mm, b=5.6mm, c=0.37mm,d=O.l5mm, e=2.6mm, f = l nun,
g=3.9mm.
Experimental results and discussion: For reference purposes, an
antenna (REF-Ant) with an unmatched 5 0 R line-feed was made
and measured. The -10 dB bandwidth for the REF-Ant and the
antenna with GT (GT-Ant) is 4 and IS%, respectively. It is worth
noting the double dips in the measurement results, the same phenomenon was observed in other papers [ I , 2, 4, 61. It was suggested by
Gauthier et al. [5] that the cavity around the antennas can be designed
to resonate close to the microstrip antenna resonance, and therefore,
ELECTRONICS LETTERS 29th August 2002 Vol. 38 No. 18
1005
resulting in an increase in bandwidth. Good radiation patterns are
obtained at frequency of 25 GHz, as shown in Fig. 2h. Similar results
are observed on E- and H-planes where both have a maximum at 0”
and decrease symmetrically to either side. Cross-polarisation pattems
are well below -20 dB.
Conclusion: A structure comprising polyimide tape on a laser-cut
tapered metal ground plane has been utilised as an effective prototype
for the suspended microstrip patch antenna. Fabrication of the antenna
on such a structure is simple and compatible with standard semiconductor etching process. A novel impedance matching design on
planar antenna structure is presented where a 18% fractional bandwidth at 25GHz is obtained. GT-Ant design is useful when high
index materials or relatively thick air substrates are used in the
designs which cause significant difference in the width of transmission lines on different layers. In addition, it also benefits in reduced
radiation loss from the transmission line discontinuity.
Acknowledgment: The authors would like to thank M. Schmidt from
LPRC, UMIST, for assistance in fabricating the GT prototype.
0 IEE 2002
Electronics Letters Online No: 20020712
DOI: 10.1049/e1:200207I2
10 June 2002
N.U. Lau, C.M. Gregory and R. Sloan (Microwave Engineering
Group, Department of Electrical Engineering and Electronics,
UMISZ RO. Box 88, Manchester M60 lQD, United Kingdom)
E-mail: mchpjn12@fs4.umist.ac.uk
Reference s
BARKER, S.J., KOT, J.S., and NIKOLIC,N.: ‘A study of the application ofsilica
aerogels in broadband millimetre-wave planar antennas’. IEEE Int.
Antennas Propagation. Symp. Dig., Atlanta, Georgia, US, June 1998,
Vol. 36, pp. 1116-1119
ZHENG, M., CHEN, Q , HALL, P.S., and FUSCO, VF.: ‘Broadband microstrip
patch antenna on micromachined silicon substrates’, Electron. Lett.,
1998, 34, ( I ) , pp. 3 4
CHANG, E-S., and WONG, K.-L.: ‘Broadband patch antenna edge-fed by
a coplanar probe feed’, Microw. Opt. Techno1 Left., 2001, 31, (4),
pp. 287-289
PAPAPOLYMEROU, I., FRANKLIN, D.R., and KATEHI, L.P.B.: ‘Micromachined
patch antennas’, IEEE Trans. Antennas Propag., 1998,46, (2), pp. 275283
GAUTHIER, G.P., RASKIN, J.P., KATEHI, L.P.B., and REBEIZ, G.M.: ‘A 94-GHz
aperture-coupled micromachined microstrip antenna’, IEEE Trans.
Antennas Propag., 1999, 47, (12), pp. 1761-1766
YOOK, L G . , and KATEHI, L.P.B.:‘Micromachined microstrip patch antenna
with controlled mutual coupling and surface waves’, IEEE Trans.
Antennas Propag., 2001,49, (9), pp. 1282-1289
MASOT, F., MEDINA, F., and HORNO, M.: ‘Analysis, synthesis, and
experimental validation of a new type of microstrip transition’, IEEE
Trans. Microw. Theory Tech, 1995, 43, (l), pp. 21-25
MASOT, E, MEDINA, E, and HORNO, M.: ‘Analysis and experimental
validation of a type of three-microstrip directional coupler’, IEEE
Trans. Microw. Theory Tech, 1994, 42, (9), pp. 1624-1631
hundred watts. A loop with a copper tube diameter >20 to 30 mm can
be capacitatively tuned over a ten-to-one frequency range at >go%
efficiency. The tube diameter requirement means that efficient (transmitting) loops do not scale with frequency and are not useful above
* 30-50 MHz. Typically a balanced loop has a small operational
bandwidth, roughly proportional to loop size, corresponding to a Q
factor of 200-600.
The classic formula 3.12 x 104[A2/A2]2 for the series radiation
resistance of a small single tum tuned loop antenna is based on
theory that is well founded and provably correct [l]. However, measurements show that it predicts radiation resistances which for loops
4 / 1 6 0 in diameter are about a thousand times less than measured
values. This shows that other radiation modes are more dominant in
practice.
It has been proposed [2] that the small tuned loop also has a folded
dipole radiation resistance, varying as the square of frequency, and this
is larger than the classic loop mode at frequencies below the loop self
resonant frequency. It has been shown by simulation [ 3 ] that a loop
radiating an electric dipole or monopole mode together with a loop
mode would be uni-directional to some extent. Practical measurements
and published loop manufacturers claims [4] have since confimied this.
Practical impedance measurements show that there are a number of
other radiation modes and loss mechanisms with different frequency
laws. Of these we find that the dominant radiation term for practical
loops has a series radiation resistance proportional to frequency and
loop area, with a magnitude that always exceeds the ‘classic’ loop
radiation resistance below the loop self-resonant frequency.
The proposed model is shown in Fig. 1. It consists of a series tuned
circuit with all possible radiation and loss resistances combined into a
single total series resistance RtOt.A novel method of combination is
proposed as follows. A gamma match is tapped on to the loop at a
suitable point to give a near enough perfect match to 50 R. Whether
twisted around the loop conductor or not, the gamma match can be
represented by the combination of voltage feed to a tapping point and
inductive (transformer) coupling to the tuned loop. A small coupling
loop can also be modelled as inductive coupling with no tapping point.
Both of these can be shown to be equivalent to a perfect transformer
and a series inductance as indicated in Fig. 1.
-
a
b
Fig. 1 Typical loop configuration and proposed simple circuit model
a Loop configuration b Circuit model
D1 :Off
t2:reflection
Smith
is”
1 UFS
I--
Simple circuit model of small tuned loop
antenna including observable
environmental effects
M.J. Underhill and M. Harper
-
impedance
loop impedanceplot
Smith chart - (r=O 2 is 1 5 1 SWR)
a
150
Frequency swept input impedances and Q factors of different sized
balanced tuned loop antennas have been measured over their tuning
ranges. A simple circuit model, with a dominant radiation resistance
proportional to frequency and loop area, agrees well with measurements. Conductor losses, dipole modes, and environmental effects are
separately identifiable.
Fig. 2 Comparison of measured and predicted input impedance at centre
of loop tuning range (10.21 MHz)
a Predicted b Measured
Introduction: The small tuned (magnetic) loop antenna typically
consists of a single turn loop, tuned by a single capacitor, with a
subsidiary input loop or gamma match. Over the HF 1.7-30MHz
frequency range loops with diameters of 0.8-1.2 m can have radiation
efficiency of no worse than 90%, and can operate with powers of a few
Fig. 2a shows the input impedance of this equivalent circuit plotted
as a reflection coefficient against frequency on a Smith Chart for the set
of component values found for a 70 cm diameter tuned loop made from
15 mm copper tubing. The loop was tuned to a resonant frequency of
1006
ELECTRONICS LETTERS 29th August 2002
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Vol. 38 No. 18